Scaling up is one way to solve your problem
Optimizing your code is another.

Are you using PHP 7.x ?
If not, switch on it, it can boost your performance significantly.

Are you using caching?
If not, work on it. It will save your CPU usage.

How your database queries look like?
Probably you have multiple unnecessary queries which can be optimized.

Are you using single or double quotes for strings in PHP?
Double quotes parse variables inside string so if you have a lot of strings with double quotes, running code with them consume more CPU comparing to strings wrapped with single quotes.